Colony growth and the ontogeny of worker polymorphism in the fire ant,Solenopsis invicta

Summary Colony size and worker polymorphism (headwidth) were determined for fire ant colonies ranging from incipient to 12 years of age. Colonies grew approximately logistically, reaching half size between 21/2 and 31/2 yr and reaching their maximum size of about 220000 workers after 4 to 6 yr. Colony size showed strong seasonal variation. There was some evidence that growth rate may vary with food density. Incipient colonies are monomorphic and consist of small workers only, but as colonies grow, production of larger workers causes the size-frequency distributions to become strongly skewed. These skewed distributions were shown to consist of two slightly overlapping normal distributions, a narrow one defined as the minor workers, and a much broader one defined as the major workers. Major workers differ from minor workers in having been subjected to a discrete, additional stimulation of body growth, resulting in a second normal subpopulation. The category of media is seen to be developmentally undefined. The mean headwidth of the workers in both of these subpopulations increased during the first 6 mo. of colony life, until colonies averaged about 4000 workers. Headwidth of minors declined somewhat in colonies older than about 5 yr, but that of majors remained stable. When the first majors appear, their weight averages about twice that of minors. This increases to about 4 times at 6 mo. and remains stable thereafter. The range of weights of majors is up to 20 times that of minors. Growth of the subpopulation of major workers is also logistic, but more rapid than the colony as a whole, causing the proportion of major workers to increase with colony size. In full sized colonies, about 35% of the workers are majors. Total biomass investment in majors increases as long as colonies grow, beginning at about 10% at 2 months and reaching about 70% in mature colonies. This suggests that major workers play an important role in colony success. The total dry biomass of workers peaked at about 106 g, that of majors at about 72 g. These values then fluctuate seasonally in parallel to number of workers. When colony growth ceases, the proportion of majors remains approximately stable. Colony size explained 98% of the variation in the number of major workers.This is paper No. 18 of the Fire Ant Research Team     

Common Patterns of Ecosystem Breakdown Under Stress

Ecosystems, despite their diversity, respond to stress in similar ways. The major pressures which cause the transformation of systems from healthy states to pathological states are classified into four groups: physical restructuring, overharvesting, waste residuals, and the introduction of non-native species. Signs of Ecosystem Distress Syndrome (EDS) are briefly examined in three contrasting ecosystems: desert grasslands, the Great Lakes, and the Baltic Sea. The issue is raised as to the difficulty in discerning between healthy ecosystems, recovering from a natural disturbance, and those ecosystems that have lost their original resilience due to anthropogenic stress. Knowledge of site history and a rigourous monitoring program are important in the evaluation of EDS. An assessment of how ecosystem services are affected is indicative of the consequences to the human component of ecosystems. Management strategies which are employed to mitigate the signs of EDS are usually initiated after resilience is lost or the ecosystem has transformed to an alternate, stress-induced, stable state. It is proposed that preventive strategies measure signs of EDS that serve as early warning signals, combined with "fitness tests" that measure ecosystem response to natural perturbations. The fitness test for ecosystems is based on the premise that unstressed systems are more resilient to natural disturbances than stressed systems.     

Persistence of Viruses by qPCR Downstream of Three Effluent-Dominated Rivers in the Western United States

This study was designed to determine the quantitative polymerase chain reaction (qPCR) signal persistence of viruses in three effluent-dominated streams. Samples were collected from the effluent outfall of three wastewater treatment plants in the Western United States and downstream at different locations. All samples were tested for the presence of pepper mild mottle virus (PMMoV), adenoviruses, norovirus GI and GII, Aichi virus, and enteroviruses using qPCR. PMMoV was detected most frequently in 54/57 (94.7%) samples, followed by adenoviruses which was detected in 21/57 (36.8%) samples. PMMoV was detected at all locations downstream and up to 32 km from the discharge point. This study demonstrated that the detection signal of PMMoV was able to persist in wastewater discharges to a greater degree than human enteric viruses in effluent-dominated rivers.